Light emitting element, light emitting device, and electronic device

ABSTRACT

It is an object of the present invention to provide a functional layer for protecting a light emitting element from being deteriorated by a physical or chemical influence when the light emitting element is manufactured or driven, and to attain extension of lifetime of an element and improvement of element characteristics without increasing a drive voltage and degrading transmittance and color purity by providing such a functional layer. One feature of the present invention is to provide a buffer layer made of a composite material for a light emitting element including aromatic hydrocarbon containing at least one vinyl skeleton and metal oxide in part of a light emitting substance containing layer, in the light emitting element fowled by interposing the light emitting substance containing layer between a pair of electrodes. The composite material for a light emitting element for forming the buffer layer of the present invention has high conductivity and is superior in transparency.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a current excitation type lightemitting element in which a light emitting substance is interposedbetween a pair of electrodes, a light emitting device having such alight emitting element, and an electronic device.

2. Description of the Related Art

A light emitting element having features such as thin shape,lightweight, and rapid response is expected to be applied to flat paneldisplays of the next generation. In addition, it is said that a lightemitting device in which light emitting elements are arranged in matrixis superior to a conventional liquid crystal display device in viewingangle and visibility.

A light emitting element is formed by interposing a light emittingsubstance containing layer between a pair of electrodes (an anode and acathode), and it is said that emission mechanism thereof is as follows:when a voltage is applied between both electrodes, holes injected froman anode and electrons injected from a cathode are recombined in a lightemitting layer in the light emitting substance containing layer, therebyforming a molecular exciton by recombination in an emission center, andenergy is released when the molecular exciton returns to a ground stateto emit light. By such a mechanism, such a light emitting element isreferred to as a current excitation type light emitting element. Asinglet excitation state and a triplet excitation state can be given astypes of an excitation state formed by a light emitting substance. Lightemission from a singlet excitation state is referred to as fluorescenceand light emission from a triplet excitation state is referred to asphosphorescence.

There are problems such that these light emitting substances are easilydeteriorated by a physical or chemical influence such as heat, moisture,and oxygen, and are easily deteriorated when the light emitting elementis manufactured or driven because these light emitting substances aregenerally formed in a thin film state with 50 nm or less in a case of asingle layer. The prevention of such deterioration of the light emittingelement leads to improvement of yield and reliability of the lightemitting element.

In a case where such a light emitting element is applied to a lightemitting device, an electronic device, and the like, the reduction ofpower consumption is also given as one of the objects. It is importantto reduce a drive voltage of a light emitting element in order to reducepower consumption.

In a current excitation type light emitting element, emission intensityis determined by an amount of flowing current. Therefore, it isnecessary to flow a large amount of current at a low voltage in order toreduce a drive voltage.

Previously, as a method for reducing a drive voltage, an approach ofproviding a buffer layer between an electrode and a layer including anorganic compound having a light emitting property, has been attempted.For example, it is known that a drive voltage can be reduced byproviding a buffer layer which is made of polyaniline (PANI) doped withcamphorsulfonic acid, between indium tin oxide (ITO) and a lightemitting layer (for example, refer to Non-Patent Document 1: Y. Yang, etal. Applied Physics Letters, Vol. 64 (10), 1245-1247 (1994)). It isexplained that this is because of the excellent carrier injectingproperty of PANI to the light emitting layer. Note that in theNon-Patent Document 1, PANI which is the buffer layer is also consideredto be a part of an electrode.

However, as described in the Non-Patent Document 1, PANI has a problemthat transmittance becomes lower when a film thickness becomes thick.Specifically, it is reported that, in a film thickness of about 250 nm,the transmittance is less than 70%. In other words, since the problem iswith the transparency of the material itself which is used for thebuffer layer, light that is generated within an element cannot beextracted efficiently.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a functional layerfor protecting a light emitting element from being deteriorated by aphysical or chemical influence when the light emitting element ismanufactured or driven, and to attain extension of lifetime of anelement and improvement of element characteristics without increasing adrive voltage and degrading transmittance and color purity by providingsuch a functional layer. Further, it is another object of the presentinvention to provide a light emitting device having such a lightemitting element and an electronic device having the light emittingdevice.

One feature of the present invention is to provide a buffer layer madeof a composite material for a light emitting element including aromatichydrocarbon containing at least one vinyl skeleton and metal oxide inpart of a light emitting substance containing layer, in the lightemitting element formed by interposing the light emitting substancecontaining layer between a pair of electrodes. The composite materialfor a light emitting element for forming the buffer layer of the presentinvention has high conductivity and is superior in transparency.

One structure of the present invention is a light emitting elementformed by interposing a light emitting substance containing layerbetween a pair of electrodes, in which a layer including aromatichydrocarbon having at least one vinyl skeleton and metal oxide isprovided in part of the light emitting substance containing layer.

Another structure of the present invention is a light emitting elementformed by interposing a light emitting substance containing layerbetween a pair of electrodes, in which a layer including aromatichydrocarbon having at least one vinyl skeleton and metal oxide isprovided in part of the light emitting substance containing layer and tobe in contact with one or both of the pair of electrodes.

Another structure of the present invention is a light emitting elementformed by interposing a light emitting substance containing layerbetween an anode and a cathode, in which a buffer layer and an electrongenerating layer are included in part of the light emitting substancecontaining layer; the buffer layer includes aromatic hydrocarbon havingat least one vinyl skeleton and metal oxide; the electron generatinglayer includes an electron transporting substance or a bipolarsubstance, and a substance of alkali metal, alkaline earth metal, alkalimetal oxide, alkaline earth metal oxide, alkali metal fluoride, oralkaline earth metal fluoride; and the buffer layer is formed betweenthe cathode and the electron generating layer to be in contact with eachof them.

Another structure of the present invention is a light emitting elementformed by interposing a light emitting substance containing layerbetween an anode and a cathode, in which at least a buffer layer and anelectron generating layer which are formed without being in contact withthe anode and the cathode are included in part of the light emittingsubstance containing layer; the buffer layer includes aromatichydrocarbon containing at least one vinyl skeleton and metal oxide; theelectron generating layer includes an electron transporting substance ora bipolar substance, and a substance of alkali metal, alkaline earthmetal, alkali metal oxide, alkaline earth metal oxide, alkali metalfluoride, or alkaline earth metal fluoride; and the electron generatinglayer is formed to be closer to the cathode side than the buffer layer,and to be in contact with the buffer layer.

In each of the structures, the metal oxide is any of molybdenum oxide,vanadium oxide, ruthenium oxide, and rhenium oxide.

In each of the structures, the aromatic hydrocarbon containing at leastone vinyl skeleton is represented by, for example, any of the followingstructural formulas (1) to (5).

In each of the structures, the aromatic hydrocarbon containing at leastone vinyl skeleton has hole mobility of 1×10⁻⁶ cm²/Vs or more.

A light emitting device having the above-described light emittingelement and an electronic device having the light emitting device arealso within the present invention. A light emitting device in thepresent specification denotes an image display device, a light emittingdevice, or a light source (including a lighting device). Further, thefollowing are all included in a light emitting device: a module in whicha connector, for example, an FPC (flexible printed circuit), a TAB (tapeautomated bonding) tape, or a TCP (tape carrier package) is attached toa light emitting device; a module provided with a printed wiring boardat the end of the TAB tape or the TCP; and a module in which an IC(integrated circuit) is directly mounted to a light emitting element bya COG (chip on glass) method.

According to the present invention, by providing a buffer layer in partof a light emitting substance containing layer of a light emittingelement, hole density in the light emitting substance containing layercan be enhanced; therefore, conductivity is increased and a drivevoltage of the light emitting element can be reduced.

Further, aromatic hydrocarbon containing at least one vinyl skeleton andmetal oxide are mixed in a composite material for a light emittingelement for forming a buffer layer of the present invention; therefore,crystallization of aromatic hydrocarbon containing at least one vinylskeleton which tends to be easily crystallized can be suppressed.

Furthermore, aromatic hydrocarbon containing at least one vinyl skeletonhas steric hindrance because of its structure; therefore, a stable filmwhich is in an amorphous state can be formed. In addition, theabove-described material has an absorption peak on a shorter wavelengthside than a visible region; thus, the material can be used withoutgiving little influence on color purity of light emitted from a lightemitting element.

Therefore, even in a case where a buffer layer is formed to be thick toprotect a light emitting layer of a light emitting element, transparencyis not degraded and increase of a drive voltage can be suppressed;therefore, deterioration and a short circuit of a light emitting elementcan be prevented and color purity can be improved by optical design.

These and other objects, features and advantages of the presentinvention will become more apparent upon reading of the followingdetailed description along with the accompanied drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are views showing an element structure of a lightemitting element of the present invention;

FIGS. 2A and 2B are views showing an element structure of a lightemitting element of the present invention;

FIG. 3 is a view showing an element structure of a light emittingelement of the present invention;

FIGS. 4A and 4B are views showing an element structure of a lightemitting element of the present invention;

FIGS. 5A and 5B are views showing a light emitting device of the presentinvention;

FIGS. 6A to 6E are views showing an electronic device of the presentinvention;

FIG. 7 is a view showing an element structure of a light emittingelement explained in Embodiment 1;

FIG. 8 is a graph showing element characteristics of a light emittingelement;

FIG. 9 is a graph showing element characteristics of a light emittingelement;

FIG. 10 is a graph showing element characteristics of a light emittingelement; and

FIG. 11 is a graph showing element characteristics of a light emittingelement.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiment modes of the present invention will be explainedin detail below with reference to the accompanied drawings or the like.However, the present invention can be implemented in variousembodiments, and it is easily understood by those skilled in the artthat embodiments and details can be modified in various ways withoutdeparting from the purpose and the scope of the present invention.Therefore, it should be noted that the present invention should not beinterpreted as being limited to the description of the embodiment modes.

Embodiment Mode 1

FIGS. 1A and 1B show an element structure of a light emitting elementwhich will be explained in this embodiment mode. Basically, a structureis employed, in which a light emitting substance containing layer 102 isinterposed between a pair of electrodes (a first electrode 101 and asecond electrode 103) as shown in FIG. 1A. In this structure, the lightemitting substance containing layer 102 has at least a light emittinglayer 105 and a buffer layer 104 made of a composite material for alight emitting element of the present invention, and the buffer layer104 is provided to be in contact with the first electrode 101. InEmbodiment Mode 1, a case where the first electrode 101 serves as ananode and the second electrode 103 serves as a cathode will beexplained. In this embodiment mode, a structure in which a holetransporting layer, an electron transporting layer, an electroninjecting layer, a hole blocking layer, and the like are appropriatelycombined in addition to the above-described structure may also beemployed.

In the present invention, the composite material for a light emittingelement is formed by aromatic hydrocarbon containing at least one vinylskeleton and metal oxide.

Aromatic hydrocarbon containing at least one vinyl skeleton isrepresented by, for example, any of the following structural formulas(1) to (5).

The aromatic hydrocarbon containing at least one vinyl skeletondescribed above preferably has hole mobility of 1×10⁻⁶ cm²/Vs or more.It is to be noted that a TOF method (time-of-flight method) utilizingtransient photocurrent can be employed to measure hole mobility here.

As the metal oxide described above, metal oxide which shows anelectron-accepting property to the aromatic hydrocarbon containing atleast one vinyl skeleton described above is preferable. As such metaloxide, for example, molybdenum oxide, vanadium oxide, ruthenium oxide,rhenium oxide, and the like are given. In addition to this, metal oxidesuch as titanium oxide, chromium oxide, zirconium oxide, hafnium oxide,tantalum oxide, tungsten oxide, or silver oxide can be used.

Metal oxide is preferably included in the buffer layer 104 so that amass ratio is 0.5 to 2 or a molar ratio is 1 to 4 with respect toaromatic hydrocarbon containing at least one vinyl skeleton (=metaloxide/aromatic hydrocarbon). As described above, by mixing aromatichydrocarbon containing at least one vinyl skeleton and metal oxide inthe buffer layer 104, crystallization of aromatic hydrocarbon containingat least one vinyl skeleton which tends to be easily crystallized can besuppressed. Further, the buffer layer 104 including aromatic hydrocarboncontaining at least one vinyl skeleton and metal oxide has littleabsorption in a visible wavelength region. In addition, absorption by anaromatic hydrocarbon containing at least one vinyl skeleton is shiftedtoward shorter wavelength in comparison to aromatic hydrocarbon withoutcontaining vinyl skeleton, because conjugation of an aromatichydrocarbon containing at least one vinyl skeleton can not be extended.Therefore, A visible light emitted from the light emitting layer 105 canbe prevented from absorption of the buffer layer 104.

In particular, among the metal oxide described above, molybdenum oxideis easily crystallized when a layer is formed using only molybdenumoxide; however, by mixing molybdenum oxide with aromatic hydrocarbon,crystallization can be suppressed in the same manner. By mixing aromatichydrocarbon and metal oxide as described above, crystallization isdisturbed by aromatic hydrocarbon and metal oxide mutually, andaccordingly, a layer which is not easily crystallized can be formed.

The buffer layer 104 can be formed to have a thickness of 50 nm or morebecause of its high conductivity.

As an anode material used for the first electrode 101, metal, alloy, anelectrically conductive compound, a mixture of these, or the like havinga high work function (a work function of 4.0 eV or more) is preferablyused. As a specific example of the anode material, gold (Au), platinum(Pt), titanium (Ti), nickel (Ni), tungsten (W), chromium (Cr),molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd),nitride of a metal material (for example, TiN), or the like can be usedbesides indium tin oxide (ITO), indium zinc oxide (IZO) in which indiumoxide is mixed with 2 to 20[%] zinc oxide (ZnO). In a case of thisembodiment mode, the buffer layer 104 is provided to be in contact withthe first electrode; therefore, aluminum (Al) or magnesium (Mg) which isgenerally known as a material having a low work function can also beused. This is because an ohmic contact with respect to an electrodematerial having a wide-range work function can be formed by providingthe buffer layer 104.

On the other hand, as a cathode material used for the second electrode103, metal, alloy, an electrically conductive compound, a mixture ofthese, or the like having a low work function (a work function of 3.8 eVor lower) is preferably used. As a specific example of the cathodematerial, transition metal including rare-earth metal can be usedbesides an element belonging to Group 1 or Group 2 in the periodic law,namely alkali metal such as Li or Cs and alkaline earth metal such asMg, Ca, or Sr, and alloy (Mg:Ag or Al:Li) or a compound (LiF, CsF, orCaF₂) containing these elements. However, in a case where a layer madeof a material which increases an electron injecting property is providedin part of the light emitting substance containing layer 102 so as to bein contact with the second electrode 103, or provided as part of thesecond electrode 103, a material having a high work function such asindium tin oxide, indium tin oxide containing silicon oxide, indiumoxide containing 2 to 20% zinc oxide, gold (Au), platinum (Pt), nickel(Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt(Co), copper (Cu), palladium (Pd), or tantalum nitride can also be used.

An anode and a cathode are made of the anode material and cathodematerial described above, respectively, by forming a thin film by adeposition method, a sputtering method, or the like. Each of the anodeand cathode preferably has a thickness of 5 to 500 nm.

The light emitting element of the present invention has a structure inwhich light generated by recombination of carriers in the light emittinglayer 105 is emitted outside from one or both of the first electrode(anode) 101 and the second electrode (cathode) 103. In other words, thefirst electrode 101 is made of a material having a light transmittingproperty in a case where light is made to be emitted from the firstelectrode 101 side, the second electrode 103 is made of a materialhaving a light transmitting property in a case where light is made to beemitted from the second electrode 103 side, and both of the electrodesare made of a material having a light transmitting property in a casewhere light is made to be emitted from both sides of the electrodes.

In Embodiment Mode 1, the light emitting substance containing layer 102which is formed between the pair of electrodes can be formed bycombining a hole transporting layer 106, an electron transporting layer107, and an electron injecting layer 108 in addition to the lightemitting layer 105 and the buffer layer 104 as shown in FIG. 1B. Here,these layers will be explained hereinafter in detail.

The hole transporting layer 106 is a layer superior in a holetransporting property, and specifically, the hole transporting layer 106is preferably made of a hole transporting substance which shows holemobility of 1×10⁻⁶ cm²/Vs or more or a bipolar substance. The holetransporting substance denotes a substance having higher hole mobilitythan electron mobility, and preferably, a substance having a value of aratio of the hole mobility to the electron mobility (=holemobility/electron mobility) of more than 100.

As the hole transporting substance, for example, an aromatic amine-based(namely a substance having a bond of benzene ring—nitrogen) compound ispreferable. As a substance which is widely used, for example,4,4′-bis[N-(3-methylphenyl)-N-phenylamino]biphenyl (hereinafter,referred to as TPD); 4,4′-bis[N-(1-naphtyl)-N-phenyl-amino]biphenyl(hereinafter, referred to as NPB) which is a derivative thereof; a starburst aromatic amine compound such as4,4′,4″-tris(N-carbazolyl)triphenylamine (hereinafter, referred to asTCTA), 4,4′,4″-tris(N,N-diphenyl-amino)-triphenylamine (hereinafter,referred to as TDATA), or4,4′,4″-tris[N-(3-methylphenyl)-N-phenyl-amino]triphenylamine(hereinafter, referred to as MTDATA) is given.

The bipolar substance denotes a substance which is described as follows:when mobility of an electron and mobility of a hole are compared witheach other, a value of a ratio of mobility of one carrier to mobility ofthe other carrier is 100 or less, preferably 10 or less. As the bipolarsubstance, for example, 2,3-bis(4-diphenylaminophenyl)quinoxaline(abbreviation: TPAQn);2,3-bis{4-[N-(1-naphthyl)-N-phenylamino]phenyl}-dibenzo[f,h]quinoxaline(abbreviation: NPADiBzQn); and the like are given. In particular, amongbipolar substances, a substance having hole mobility and electronmobility of 1×10⁻⁶ cm²/Vs or more is preferably used.

The light emitting layer 105 includes at least one kind of lightemitting substance, and the light emitting substance described heredenotes a substance which has favorable luminous efficiency and can emitlight of a desired wavelength. The light emitting layer may be a layermade of only a light emitting substance; however, a layer in which alight emitting substance is mixed to be dispersed in a layer made of asubstance having larger energy gap (energy gap between the LUMO leveland the HOMO level) than that of a light emitting substance may also beused. It is to be noted that light emission can be prevented from beingquenched due to concentration by dispersing a light emitting substancewhich serves as a guest (also referred to as a guest substance) in alight emitting substance which serves as a host (also referred to as ahost substance) in the light emitting layer.

As a specific light emitting substance, the following variousfluorescent pigments are effective: tris(8-quinolinolato)aluminum(hereinafter, referred to as Alq₃);tris(4-methyl-8-quinolinolato)aluminum (hereinafter, referred to asAlmq₃); bis(10-hydroxybenzo[h]-quinolinato)beryllium (hereinafter,referred to as BeBq₂);bis(2-methyl-8-quinolinolato)-(4-hydroxy-biphenylyl)-aluminum(hereinafter, referred to as BAIq);bis[2-(2-hydroxyphenyl)-benzoxazolato]zinc (hereinafter, referred to asZn(BOX)₂); bis[2-(2-hydroxyphenyl)-benzothiazolato]zinc (hereinafter,referred to as Zn(BTZ)₂);4-dicyanomethylene-2-isopropyl-6-[2-(1,1,7,7-tetramethyl-9-julolidyl)ethenyl]-4H-pyran(hereinafter, referred to as DCJTI);4-dicyanomethylene-2-methyl-6-[2-(1,1,7,7-tetramethyl-9-julolidyl)ethenyl]-4H-pyran(hereinafter, referred to as DCJT);4-dicyanomethylene-2-tert-butyl-6-[2-(1,1,7,7-tetramethyl-9-julolidyl)ethenyl]-4H-pyran(hereinafter, referred to as DCJTB); periflanthene;2,5-dicyano-1,4-bis[2-(10-methoxy-1,1,7,7-tetramethyl-9-julolidyl)ethenyl]benzene;N,N′-dimethylquinacridone (hereinafter, referred to as DMQd); coumarin6; coumarin 545T; 9,10-bis(2-naphthyl)-tert-butylanthracene(hereinafter, referred to as t-BuDNA); 9,9′-bianthryl;9,10-diphenylanthracene (hereinafter, referred to as DPA);9,10-bis(2-naphthyl)anthracene (hereinafter, referred to as DNA);bis(2-methyl-8-quinolinolato)-4-phenylphenolato-gallium (hereinafter,referred to as BGaq);bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (hereinafter,referred to as BAIq); tris(2-phenylpyridine)iridium (hereinafter,referred to as Ir(ppy)₃);2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin-platinum (hereinafter,referred to as PtOEP);bis[2-(3,5-bis(trifluoromethyl)phenyl)pyridinato-N,C^(2′)]iridium(III)picolinate (hereinafter, referred to as Ir(CF₃ ppy)₂(pic));bis[2-(4,6-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)acetylacetonate(hereinafter, referred to as FIr(acac));bis[2-(4,6-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)picolinate(hereinafter, referred to as FIr(pic)); and the like.

In a case where the light emitting layer is formed by combining a hostsubstance and a guest substance, the light emitting layer may be formedby combining the light emitting substance described above and a hostsubstance which will be described hereinafter.

As a specific host substance, the following can be used:tris(8-quinolinolato)aluminum (hereinafter, referred to as Alq₃);tris(4-methyl-8-quinolinolato)aluminum (hereinafter, referred to asAlmq₃); bis(10-hydroxybenzo[h]-quinolinato)beryllium (hereinafter,referred to as BeBq₂);bis(2-methyl-8-quinolinolato)-(4-hydroxy-biphenylyl)-aluminum(hereinafter, referred to as BAlq);bis[2-(2-hydroxyphenyl)-benzoxazolato]zinc (hereinafter, referred to asZn(BOX)₂); bis[2-(2-hydroxyphenyl)-benzothiazolato]zinc (hereinafter,referred to as Zn(BTZ)₂); 9,10-bis(2-naphthyl)-tert-butylanthracene(hereinafter, referred to as t-BuDNA); 9,10-bis(2-naphthyl)anthracene(hereinafter, referred to as DNA);bis(2-methyl-8-quinolinolato)-4-phenylphenolato-gallium (hereinafter,referred to as BGaq); 4,4′-di(N-carbazolyl)biphenyl (hereinafter,referred to as CBP); 4,4′,4″-tri(N-carbazolyl)triphenylamine(hereinafter, referred to as TCTA);2,2′,2″-(1,3,5-benzenetriyl)-tris(1-phenyl-1H-benzimidazole)(hereinafter, referred to as TPBI); TPAQn; and the like.

The electron transporting layer 107 is a layer which is superior in anelectron transporting property, and specifically, the electrontransporting layer 107 is preferably made of an electron transportingsubstance which shows electron mobility of 1×10⁻⁶ cm²/Vs or more or abipolar substance. The electron transporting substance denotes asubstance having higher electron mobility than hole mobility, andpreferably, a substance having a value of a ratio of the electronmobility to the hole mobility (=electron mobility/hole mobility) of morethan 100.

As a specific electron transporting substance, a metal complex having aquinoline skeleton or a benzoquinoline skeleton such as Alq₃, Almq₃, orBeBq₂ which is described above; BAIq which is a mixed ligand complex; orthe like is preferable. In addition, a metal complex having anoxazole-based or thiazole-based ligand such as Zn(BOX)₂ or Zn(BTZ)₂ canalso be used. Further, an oxadiazole derivative such as2-(4-biphenylyl)-5-(4-tert-buthylphenyl)-1,3,4-oxadiazole (hereinafter,referred to as PBD) or1,3-bis[5-(p-tert-buthylphenyl)-1,3,4-oxadiazol-2-yl]benzene(hereinafter, referred to as OXD-7); a triazole derivative such as3-(4-tert-buthylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(hereinafter, referred to as TAZ) or3-(4-tert-buthylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(hereinafter, referred to as p-EtTAZ); a phenanthroline derivative suchas bathophenanthroline (hereinafter, referred to as BPhen) orbathocuproin (hereinafter, referred to as BCP); and, in addition,4,4-bis(5-methylbenzoxazolyl-2-yl)stilbene (hereinafter, referred to asBzOs); or the like can be used as well as a metal complex. It is to benoted that the materials described above can be used as the bipolarsubstance.

The electron injecting layer 108 is a layer having a function to assistelectrons to be injected from the second electrode 103. In a case of astructure shown in FIG. 1B, the electron injecting layer 108 has afunction to assist electrons to be injected from the second electrode103 to the electron transporting layer 107. By providing the electroninjecting layer 108, a difference between a work function of the secondelectrode 103 and electron affinity of the electron transporting layer107 is relieved; thus, electrons are easily injected. The electroninjecting layer 108 is preferably made of a substance of which electronaffinity is higher than that of a substance for forming the electrontransporting layer 107 and lower than that of a work function of asubstance for forming the second electrode 103. Alternatively, theelectron injecting layer 108 is preferably made of a substance having aneffect of curving energy band by being provided as a thin film of about1 nm to 2 nm between the electron transporting layer 107 and the secondelectrode 103.

As a specific example of a substance which can be used for forming theelectron injecting layer 108, the following are given: an inorganicsubstance of alkali metal such as lithium (Li); alkaline earth metalsuch as magnesium (Mg); fluoride of alkali metal such as cesium fluoride(CsF); fluoride of alkaline earth metal such as calcium fluoride (CaF₂);oxide of alkali metal such as lithium oxide (Li₂O), sodium oxide (Na₂O),or potassium oxide (K₂O); or oxide of alkaline earth metal such ascalcium oxide (CaO) or magnesium oxide (MgO). These substances arepreferable because there is an effect of curving the energy band bybeing provided as a thin film.

In addition to the inorganic substance, an organic substance which canbe used for forming the electron transporting layer 107, such asbathophenanthroline (abbreviation: BPhen); bathocuproin (abbreviation:BCP);3-(4-tert-buthylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: p-EtTAZ); or3-(4-tert-buthylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: TAZ), can also be used as a substance for Miming theelectron injecting layer 108 by selecting a substance of which electronaffinity is higher than that of a substance which is used for formingthe electron transporting layer 107 among these substances. In otherwords, the electron injecting layer 108 may be formed by selecting asubstance so that electron affinity in the electron injecting layer 108is higher than that in the electron transporting layer 107. It is to benoted that the second electrode 103 is preferably made of a substancehaving a low work function such as aluminum in a case of providing theelectron injecting layer 108.

In addition, an electron generating layer can also be provided insteadof the electron injecting layer 108. The electron generating layer is alayer for generating electrons, which can be formed by mixing at leastone substance of an electron transporting substance and a bipolarsubstance with a substance (donor) which shows an electron-donatingproperty to these substances. It is to be noted that a substance havingelectron mobility of 1×10⁻⁶ cm²/Vs or more is preferable as the electrontransporting substance and the bipolar substance.

As for the electron transporting substance and the bipolar substance,the above substances can be used for each. In addition, as for thesubstance which shows an electron-donating property, a substance ofalkali metal or alkaline earth metal, specifically, lithium (Li),calcium (Ca), sodium (Na), potassium (K), magnesium (Mg), or the likecan be used. Moreover, a substance such as alkali metal oxide, alkalineearth metal oxide, alkali metal nitride, or alkaline earth metalnitride, specifically, lithium oxide (Li₂O), calcium oxide (CaO), sodiumoxide (Na₂O), potassium oxide (K₂O), magnesium oxide (MgO), lithiumfluoride (LiF), cesium fluoride (CsF), calcium fluoride (CaF2), or thelike can also be used as the substance which shows an electron-donatingproperty.

In addition, although not shown in FIG. 1B, a hole blocking layer can beprovided between the light emitting layer 105 and the electrontransporting layer 107. By providing the hole blocking layer, holes canbe prevented from flowing in the second electrode 103 direction withoutbeing recombined with electrons in the light emitting layer 105; thus,recombination efficiency of carriers can be increased. Moreover,excitation energy generated in the light emitting layer 105 can beprevented from moving to other layers such as the electron transportinglayer 107.

For the hole blocking layer, a substance is particularly selected, ofwhich ionization potential and excitation energy are higher than thoseof a substance which is used for forming the light emitting layer 105,among substances which can be used for forming the electron transportinglayer 107, such as BAlq, OXD-7, TAZ, or BPhen. In other words, the holeblocking layer may be formed by selecting a substance so that ionizationpotential in the hole blocking layer is higher than that in the electrontransporting layer 107. In the same manner, a layer for blockingelectrons from flowing in the second electrode 103 direction withoutbeing recombined with holes in the light emitting layer 105 may also beprovided between the light emitting layer 105 and the hole transportinglayer 106.

Accordingly, a light emitting element is obtained, which has the bufferlayer 104 made of the composite material for a light emitting element ofthe present invention in part of the light emitting substance containinglayer 102 of a light emitting element, where the buffer layer 104 isprovided to be in contact with the first electrode 101 which is ananode.

In Embodiment Mode 1, the composite material for a light emittingelement for forming the buffer layer 104 has a hole transportingproperty, can suppress crystallization of aromatic hydrocarboncontaining at least one vinyl skeleton which tends to be easilycrystallized because aromatic hydrocarbon containing at least one vinylskeleton and metal oxide are mixed, and further, has a property of hightransparency and high conductivity.

Thus, even if the buffer layer 104 is formed to be thick, transparencyis not damaged, and increase of a drive voltage can be suppressed;therefore, a thickness can be freely set to prevent a short circuit of alight emitting element and to improve color purity by optical design. Itis effective that the thickness of the buffer layer 104 is set to be 60nm or more to prevent a short circuit.

Embodiment Mode 2

In Embodiment Mode 2, a structure which is different from the lightemitting element explained in Embodiment Mode 1, specifically, astructure in which a buffer layer is provided to be in contact with asecond electrode (cathode) will be explained.

FIGS. 2A and 2B show an element structure of a light emitting elementwhich will be explained in Embodiment Mode 2. Basically, a structure isemployed, in which a light emitting substance containing layer 202 isinterposed between a pair of electrodes (a first electrode 201 and asecond electrode 203) as shown in FIG. 2A. In this structure, the lightemitting substance containing layer 202 has at least a light emittinglayer 204, an electron generating layer 205, and a buffer layer 206 madeof a composite material for a light emitting element of the presentinvention, and the buffer layer 206 is provided between the secondelectrode 203 and the electron generating layer 205 to be in contactwith each of them. In Embodiment Mode 2, a case where the firstelectrode 201 serves as an anode and the second electrode 203 serves asa cathode will be explained. In Embodiment Mode 2, a structure in whicha hole injecting layer, a hole transporting layer, an electrontransporting layer, a hole blocking layer, and the like areappropriately combined in addition to the above structure may also beemployed.

The composite material for a light emitting element for forming thebuffer layer 206 includes aromatic hydrocarbon containing at least onevinyl skeleton shown in Embodiment Mode 1 and metal oxide shown inEmbodiment Mode 1.

Also in Embodiment Mode 2, metal oxide is preferably included in thebuffer layer 206 so that a mass ratio is 0.5 to 2 or a molar ratio is 1to 4 with respect to aromatic hydrocarbon containing at least one vinylskeleton (=metal oxide/aromatic hydrocarbon). The buffer layer 206 canbe formed to have a thickness of 50 nm or more because of its highconductivity.

The electron generating layer 205 is a layer for generating electrons,which can be formed by mixing at least one substance of an electrontransporting substance and a bipolar substance with a substance (donor)which shows an electron-donating property to these substances. It is tobe noted that a substance having electron mobility of 1×10⁻⁶ cm²/Vs ormore is preferable as the electron transporting substance and thebipolar substance.

As the electron transporting substance, the bipolar substance, and thesubstance (donor) which shows an electron-donating property, which areused for forming the electron generating layer 205, the substance givenin Embodiment Mode 1 can be used for each.

As an anode material used for the first electrode 201, metal, alloy, anelectrically conductive compound, a mixture of these, or the like havinga high work function (a work function of 4.0 eV or more) can be used. Asa specific example of the anode material, gold (Au), platinum (Pt),titanium (Ti), nickel (Ni), tungsten (W), chromium (Cr), molybdenum(Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), nitride of ametal material (for example, TiN), or the like can be used besidesindium tin oxide (ITO), indium zinc oxide (IZO) in which indium oxide ismixed with 2 to 20[%] zinc oxide (ZnO).

On the other hand, as a cathode material used for the second electrode203, metal, alloy, an electrically conductive compound, a mixture ofthese, or the like can be used. As a specific example of the cathodematerial, a material having a high work function such as indium tinoxide, indium tin oxide containing silicon oxide, indium oxidecontaining 2 to 20% zinc oxide, gold (Au), platinum (Pt), nickel (Ni),tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co),copper (Cu), palladium (Pd), aluminum (Al), silver (Ag), or tantalumnitride can also be used.

An anode and a cathode are made of the anode material and cathodematerial described above, respectively, by forming a thin film by adeposition method, a sputtering method, or the like. Each of the anodeand cathode preferably has a thickness of 5 to 500 nm.

The light emitting element of the present invention has a structure inwhich light generated by recombination of carriers in the light emittingsubstance containing layer 202 is emitted outside from one or both ofthe first electrode (anode) 201 and the second electrode (cathode) 203.In other words, the first electrode 201 is made of a material having alight transmitting property in a case where light is made to be emittedfrom the first electrode 201 side, the second electrode 203 is made of amaterial having a light transmitting property in a case where light ismade to be emitted from the second electrode 203 side, and both of theelectrodes are made of a material having a light transmitting propertyin a case where light is made to be emitted from both sides of theelectrodes.

In Embodiment Mode 2, the light emitting substance containing layer 202which is formed between the pair of electrodes can be formed bycombining a hole injecting layer 207, a hole transporting layer 208, andan electron transporting layer 209 in addition to the light emittinglayer 204, the electron generating layer 205, and the buffer layer 206as shown in FIG. 2B. Here, these layers will be explained hereinafter indetail.

The hole injecting layer 207 is, in Embodiment Mode 2, a layer having afunction to assist holes to be injected from the first electrode 201 tothe hole transporting layer 208. By providing the hole injecting layer207, a difference between a work function of the first electrode 201 andionization potential of the hole transporting layer 208 is relieved;thus, holes are easily injected. The hole injecting layer 207 ispreferably made of a substance of which ionization potential is lowerthan that of a substance for fowling the hole transporting layer 208 andhigher than that of a work function of a substance for forming the firstelectrode 201.

As a specific example of a substance which can be used for forming thehole injecting layer 207, a low molecular compound such asphthalocyanine (abbreviation: H₂Pc) or copper phthalocyanine(abbreviation: CuPc), a high molecular compound such aspoly(ethylenedioxythiophene)/poly(styrene sulfonate) water solution(abbreviation: PEDOT/PSS), and the like are given.

The hole transporting layer 208 is a layer superior in a holetransporting property, and in Embodiment Mode 2, has a function totransport holes injected from the hole injecting layer 207 to the lightemitting layer 204. It is to be noted that the hole transporting layer208 is preferably made of a hole transporting substance which shows holemobility of 1×10⁻⁶ cm²/Vs or more or a bipolar substance. As the holetransporting substance and the bipolar substance herein described, thesubstance given in Embodiment Mode 1 can be used for each.

The light emitting layer 204 includes at least one kind of lightemitting substance, and a light emitting substance described heredenotes a substance which has favorable luminous efficiency and can emitlight of a desired wavelength. The light emitting layer 204 may be alayer made of only a light emitting substance; however, a layer in whicha light emitting substance is mixed to be dispersed in a layer made of asubstance having larger energy gap (energy gap between the LUMO leveland the HOMO level) than that of a light emitting substance may also beused. It is to be noted that light emission can be prevented from beingquenched due to concentration by dispersing a light emitting substancewhich serves as a guest (also referred to as a guest substance) in alight emitting substance which serves as a host (also referred to as ahost substance) in the light emitting layer. As a specific lightemitting substance which can be used for the light emitting layer 204,the substance given in Embodiment Mode 1 (including a guest substance)can be used.

The electron transporting layer 209 is a layer which is superior in anelectron transporting property, and in Embodiment Mode 2, has a functionto transport electrons injected from the electron generating layer 205to the light emitting layer 204. As a substance for forming the electrontransporting layer 209, specifically, an electron transporting substancewhich shows electron mobility of 1×10⁻⁶ cm²/Vs or more or a bipolarsubstance is preferably used. As the electron transporting substance andthe bipolar substance herein described, the substance given inEmbodiment Mode 1 can be used for each.

In addition, although not shown in FIG. 2B, a hole blocking layer can beprovided between the light emitting layer 204 and the electrontransporting layer 209. By providing the hole blocking layer, holes canbe prevented from flowing in the second electrode 203 direction withoutbeing recombined with electrons in the light emitting layer 204; thus,recombination efficiency of carriers can be increased. As the substancefor forming the hole blocking layer, the substance given in EmbodimentMode 1 can be used.

Accordingly, a light emitting element is obtained, which has the bufferlayer 206 made of the composite material for a light emitting element ofthe present invention in part of the light emitting substance containinglayer 202 of a light emitting element, where the buffer layer 206 isprovided between the second electrode 203 which is a cathode and theelectron generating layer 205 to be in contact with each of them.

In the light emitting element of Embodiment Mode 2, the compositematerial for forming the buffer layer 206 has a hole transportingproperty and can suppress crystallization of aromatic hydrocarboncontaining at least one vinyl skeleton because aromatic hydrocarboncontaining at least one vinyl skeleton and metal oxide are mixed.Further, the composite material for forming the buffer layer 206 has aproperty of high transparency and high conductivity.

Thus, even if the buffer layer 206 is formed to be thick, transparencyis not damaged, and increase of a drive voltage can be suppressed;therefore, a thickness can be freely set to prevent a short circuit of alight emitting element and to improve color purity by optical design. Itis effective that the thickness of the buffer layer 206 is set to be 60nm or more to prevent a short circuit.

In a case or the like where the second electrode 203 is formed by asputtering method after fanning the light emitting substance containinglayer 202, the buffer layer 206 serves as a barrier film, which canreduce damage to the light emitting layer 204 and the like.

Further, in the present invention, a structure shown in FIG. 3, in whicha buffer layer made of a composite material for a light emitting elementof the present invention is provided to be in contact with bothelectrodes (anode and cathode) of a light emitting element, can beformed by combining the structure shown in Embodiment Mode 2 and thestructure shown in Embodiment Mode 1.

In FIG. 3, a light emitting substance containing layer 302 is interposedbetween a pair of electrodes (a first electrode 301 and a secondelectrode 303), and the light emitting substance containing layer 302has at least a light emitting layer 305, a buffer layer (a first bufferlayer 304 and a second buffer layer 307) made of a composite materialfor a light emitting element of the present invention, and an electrongenerating layer 306.

The first buffer layer 304 is provided to be in contact with the firstelectrode 301, and the second buffer layer 307 is provided between thesecond electrode 303 and the electron generating layer 306 to be incontact with each of them. Also in this case, the first electrode 301serves as an anode, and the second electrode 303 serves as a cathode.Further, also in a case of this structure, a structure can be employed,in which a hole transporting layer, an electron transporting layer, ahole blocking layer, and the like are combined in addition to thestructure shown in FIG. 3. As a substance for forming the holetransporting layer, the electron transporting layer, and the holeblocking layer, the substance given in Embodiment Mode 1 can be used.

Also in the light emitting element shown in FIG. 3, the compositematerial for a light emitting element for forming the buffer layer (thefirst buffer layer 304 and the second buffer layer 307) has a holetransporting property, is not easily crystallized, and has a property ofhigh transparency and high conductivity. Thus, even if the first bufferlayer 304 and second buffer layer 307 are formed to be thick,transparency is not damaged and increase of a drive voltage can besuppressed; therefore, thickness of the first buffer layer 304 and thesecond buffer layer 307 can be freely set to prevent a short circuit ofa light emitting element and to improve color purity by optical design.

In a case or the like where the second electrode 303 is formed by asputtering method after forming the light emitting substance containinglayer 302, the second buffer layer 307 serves as a barrier film, whichcan reduce damage to the light emitting layer 305 and the like. Further,in a case where the first buffer layer 304 and the second buffer layer307 are formed by the same substance, both sides of the light emittingsubstance containing layer 302 is formed by the same substance;therefore, an effect that stress distortion is suppressed can beexpected.

Embodiment Mode 3

In Embodiment Mode 3, a light emitting element which has a structuredifferent from that in Embodiment Modes 1 and 2, specifically, a lightemitting element having a structure in a case of providing at least onebuffer layer formed without being in contact with an anode and acathode, which is a so-called multiphoton light emitting element, willbe explained.

FIGS. 4A and 4B show an element structure of a light emitting elementwhich will be explained in Embodiment Mode 3. Basically, a structure isemployed, in which a light emitting substance containing layer 402 isinterposed between a pair of electrodes (a first electrode 401 and asecond electrode 403) as shown in FIG. 4A. In the structure, the lightemitting substance containing layer 402 has at least a first lightemitting layer 406, a second light emitting layer 407, an electrongenerating layer 404, and a buffer layer 405 made of a compositematerial for a light emitting element of the present invention, and atleast the buffer layer 405 and the electron generating layer 404 whichis formed to be in contact therewith are provided without being incontact with the first electrode 401 and the second electrode 403. InEmbodiment Mode 3, a case where the first electrode 401 serves as ananode and the second electrode 403 serves as a cathode will beexplained. Further, in Embodiment Mode 3, a structure may be employed,in which a hole injecting layer, a hole transporting layer, an electrontransporting layer, a hole blocking layer, an electron injecting layer,and the like are appropriately combined in addition to the abovestructure.

The composite material for a light emitting element for forming thebuffer layer 405 includes aromatic hydrocarbon containing at least onevinyl skeleton shown in Embodiment Mode 1 and metal oxide shown inEmbodiment Mode 1.

Also in Embodiment Mode 3, metal oxide is preferably included in thebuffer layer 405 so that a mass ratio is 0.5 to 2 or a molar ratio is 1to 4 with respect to aromatic hydrocarbon containing at least one vinylskeleton (=metal oxide/aromatic hydrocarbon). The buffer layer 405 canbe formed to have a thickness of 50 nm or more because of its highconductivity.

In Embodiment Mode 3, the electron generating layer 404 is formed to bein contact with the buffer layer 405, and the electron generating layer404 is formed closer to the first electrode 401 side, which is an anode,than the buffer layer 405. The electron generating layer 404 is a layerfor generating electrons, which can be formed by mixing at least onesubstance of an electron transporting substance and a bipolar substancewith a substance which shows an electron-donating property to thesesubstances. Here, it is preferable to particularly use a substancehaving electron mobility of 1×10⁻⁶ cm²/Vs or more among the electrontransporting substance and the bipolar substance.

As for the electron transporting substance, the bipolar substance, andthe substance which shows an electron-donating property, which are usedfor forming the electron generating layer 404, the substance given inEmbodiment Mode 1 can be used for each.

As an anode material for the first electrode 401, metal, alloy, anelectrically conductive compound, a mixture of these, or the like havinga high work function (a work function of 4.0 eV or more) can be used. Asa specific example of the anode material, gold (Au), platinum (Pt),titanium (Ti), nickel (Ni), tungsten (W), chromium (Cr), molybdenum(Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), nitride of ametal material (for example, TiN), or the like can be used besidesindium tin oxide (ITO), indium zinc oxide (IZO) in which indium oxide ismixed with 2 to 20[%] zinc oxide (ZnO). In a case of providing a bufferlayer to be in contact with the first electrode 401 as shown inEmbodiment Mode 1, aluminum (Al) or magnesium (Mg) which is generallyknown as a material having a low work function can also be used. This isbecause an ohmic contact with respect to an electrode material having awide-range work function can be formed by providing the buffer layer405.

On the other hand, as a cathode material for the second electrode 403,metal, alloy, an electrically conductive compound, a mixture of these,or the like having a low work function (a work function of 3.8 eV orlower) can be used. As a specific example of the cathode material,transition metal containing rare-earth metal can be used besides anelement belonging to Group 1 or Group 2 in the periodic law, namelyalkali metal such as Li or Cs, alkaline earth metal such as Mg, Ca, orSr, and alloy containing these (Mg:Ag or Al:Li) or a compound (LiF, CsF,or CaF₂). However, in addition to the above structure, in a case where abuffer layer is provided to be in contact with the second electrode 403and an electron generating layer is provided to be in contact with thebuffer layer, a material or the like having a high work function such asindium tin oxide, indium tin oxide containing silicon oxide, indiumoxide containing 2 to 20% zinc oxide, gold (Au), platinum (Pt), nickel(Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt(Co), copper (Cu), palladium (Pd), or tantalum nitride can be used.

An anode and a cathode are made of the anode material and cathodematerial described above, respectively, by forming a thin film by adeposition method, a sputtering method, or the like. Each of the anodeand cathode preferably has a thickness of 5 to 500 nm.

The light emitting element of the present invention has a structure inwhich light generated by recombination of carriers in the light emittingsubstance containing layer 402 is emitted outside from one or both ofthe first electrode (anode) 401 and the second electrode (cathode) 403.In other words, the first electrode 401 is made of a material having alight transmitting property in a case where light is made to be emittedfrom the first electrode 401 side, the second electrode 403 is made of amaterial having a light transmitting property in a case where light ismade to be emitted from the second electrode 403 side, and both of theelectrodes are made of a material having a light transmitting propertyin a case where light is made to be emitted from both sides of theelectrodes.

In Embodiment Mode 3, the electron generating layer 404 and the bufferlayer 405 are formed without being in contact with the first electrode401 and the second electrode 403, and at least the first light emittinglayer 406 is provided between the first electrode 401 and the electrongenerating layer 404. Further, at least the second light emitting layer407 is provided between the buffer layer 405 and the second electrode403. As the substance used for forming the first light emitting layer406 and the second light emitting layer 407, the substance mentioned asa substance for forming the light emitting layer in Embodiment Mode 1can be used.

In Embodiment Mode 3, a buffer layer can be provided in addition to theabove structure so as to be in contact with the first electrode 401 andthe second electrode 403, respectively. In this case, a first bufferlayer 408 is provided to be in contact with the first electrode 401 anda third buffer layer 412 is provided to be in contact with the secondelectrode 403, as shown in FIG. 4B. Further, a second electrongenerating layer 411 is provided to be in contact with the third bufferlayer 412, and a first electron generating layer 409 and a second bufferlayer 410 shown in FIG. 4B have the same function as the electrongenerating layer 404 and the buffer layer 405 shown in FIG. 4A.

The first buffer layer 408, the second buffer layer 410, and the thirdbuffer layer 412 shown in FIG. 4B can be made of the composite materialfor a light emitting element including aromatic hydrocarbon containingat least one vinyl skeleton and metal oxide shown in Embodiment Mode 1.The first electron generating layer 409 and the second generating layer411 shown in FIG. 4B can be formed by mixing at least one substance ofthe electron transporting substance and the bipolar substance, and asubstance which shows an electron-donating property to these substancesshown in Embodiment Mode 1.

In FIG. 4B (although not shown here), a hole transporting layer, anelectron transporting layer, a hole blocking layer, and the like can beformed by being appropriately combined in a region between the firstbuffer layer 408 and the first electron generating layer 409, whichincludes at least the first light emitting layer 406, and a regionbetween the second buffer layer 410 and the second electron generatinglayer 411, which includes at least the second light emitting layer 407.It is to be noted that each of the hole transporting layer, the electrontransporting layer, and the hole blocking layer can be made of thesubstance mentioned in Embodiment Mode 1, respectively.

Accordingly, a light emitting element having the buffer layer 405 madeof the composite material for a light emitting element of the presentinvention in part of the light emitting substance containing layer 402of the light emitting element, in which at least the buffer layer 405and the electron generating layer 404 formed to be in contact therewithare formed without being in contact with the first electrode 401 and thesecond electrode 403, can be obtained.

In the light emitting element in Embodiment Mode 3, the compositematerial for a light emitting element for forming the buffer layer 405(in a case of being shown in FIG. 4B, the first buffer layer 408, thesecond buffer layer 410, and the third buffer layer 412) has a holetransporting property, and crystallization of aromatic hydrocarboncontaining at least one vinyl skeleton can be suppressed becausearomatic hydrocarbon containing at least one vinyl skeleton and metaloxide are mixed. Further, the composite material for a light emittingelement has a property of high transparency and high conductivity.

Thus, even if the buffer layer 405 (in a case of being shown in FIG. 4B,the first buffer layer 408, the second buffer layer 410, and the thirdbuffer layer 412) is fanned to be thick, transparency is not damaged andincrease of a drive voltage can be suppressed; therefore, a thicknesscan be freely set to prevent a short circuit of a light emitting elementand to improve color purity by optical design. It is effective that thethickness of the buffer layer 405 (in a case of being shown in FIG. 4B,the first buffer layer 408, the second buffer layer 410, and the thirdbuffer layer 412) is set to be 60 nm or more to prevent a short circuit.

As shown in FIG. 4B, in a case or the like where the second electrode403 is formed by a sputtering method after forming the light emittingsubstance containing layer 402, the third buffer layer 412 serves as abarrier film, which can reduce damage to the second light emitting layer407 and the like. Further, in a case where the first buffer layer 408and the second buffer layer 412 are formed by the same substance, bothsides of the light emitting substance containing layer 402 is formed bythe same substance; therefore, an effect that stress distortion issuppressed can be expected.

Embodiment Mode 4

In Embodiment Mode 4, a light emitting device having a light emittingelement formed by the present invention in a pixel portion will beexplained with reference to FIGS. 5A and 5B. Although a light emittingdevice having an active matrix light emitting element formed by thepresent invention is shown in FIGS. 5A and 5B, the present invention canbe applied to a light emitting device having a passive matrix lightemitting element. The light emitting device in the present inventionincludes a control means for driving a light emitting element, and thelike, in addition to the light emitting element of the presentinvention. FIG. 5A is a top view showing a light emitting device andFIG. 5B is a cross-sectional view taken along A-A′. Reference numeral501 indicated by a dotted line denotes a driver circuit portion (asource side driver circuit); 502, a pixel portion; 503, a driver circuitportion (a gate side driver circuit); 504, a sealing substrate; 505, asealing member, and 507, a space surrounded by the sealing member 505.

Reference numeral 508 denotes a wiring for transmitting a signalinputted to the source side driver circuit 501 and the gate side drivercircuit 503, and the wiring 508 receives a video signal, a clock signal,a start signal, a reset signal, and the like from an FPC (flexibleprinted circuit) 509 which is to be an external input terminal. Althoughonly the FPC is shown here, a printed wiring board (PWB) may be attachedto the FPC. The light emitting device in the present specificationincludes not only the light emitting device itself but also includes astate where the FPC or PWB is attached to the light emitting device.

Next, a cross-sectional structure will be explained with reference toFIG. 5B. Although a driver circuit portion and a pixel portion areformed on an element substrate 510, a source side driver circuit 501which is a driver circuit portion and a pixel portion 502 are shownhere.

As the source side driver circuit 501, a CMOS circuit in which ann-channel TFT 523 and a p-channel TFT 524 are combined is formed. A TFTfor forming a driver circuit may be formed by a CMOS circuit, a PMOScircuit, or an NMOS circuit. In this embodiment mode, adriver-integrated type in which a driver circuit is formed on asubstrate is shown; however, it is not necessary to have the structure,and the driver circuit can be formed not on the substrate but outside.

The pixel portion 502 is formed by a plurality of pixels including aswitching TFT 511, a current control TFT 512, and a first electrode 513which is electrically connected to a drain thereof. An insulator 514 isformed so as to cover an edge portion of the first electrode 513. Here,the insulator 514 is formed by a positive type photosensitive acrylicresin film.

A light emitting substance containing layer 516 and a second electrode517 are formed on the first electrode 513, respectively. Here, as amaterial used for the first electrode 513 serving as an anode, amaterial having a high work function is desirable. For example, astacked film such as a two-layer structure of titanium nitride and afilm containing aluminum as its main component or a three-layerstructure of a titanium nitride film, a film containing aluminum as itsmain component, and a titanium nitride film can be used besides asingle-layer film such as an ITO (indium tin oxide) film, an indium zincoxide (IZO) film, a titanium nitride film, a chromium film, a tungstenfilm, a Zn film, or a Pt film. By employing the stacked structure, aresistance as the wiring can be low, and an ohmic contact can befavorably formed, and further, the stacked structure can be made toserve as an anode.

The light emitting substance containing layer 516 is formed by adeposition method using a deposition mask or an ink jet method. Thelight emitting substance containing layer 516 includes a buffer layermade of a composite material for a light emitting element of the presentinvention. Besides, a light emitting layer, an electron generatinglayer, a hole transporting layer, an electron transporting layer, a holeblocking layer, a hole injecting layer, an electron injecting layer, andthe like are included. In a case of forming these layers, a lowmolecular-based material, an intermediate molecular material (includingoligomer and dendrimer), or a high molecular-based material can be used.In a case of forming the light emitting substance containing layer, asingle layer or a stack of an organic compound is generally used;however, the present invention also includes a structure in which aninorganic compound is used in part of a film made of an organiccompound.

In the present invention, a buffer layer is provided to be in contactwith one of the electrodes (one of an anode and a cathode) of the lightemitting element, or provided to be in contact with both of theelectrodes, or alternatively, provided without being in contact withboth of the electrodes. However, in a case where the buffer layer isformed to be in contact with the cathode, it is necessary to provide theelectron generating layer to be in contact with the surface of thebuffer layer opposite to the surface being in contact with the cathode.In a case where the buffer layer is formed without being in contact withboth of the electrodes, it is necessary to provide the electrongenerating layer on the anode side to be in contact with the bufferlayer.

In addition, the second electrode (cathode) 517 is formed on the lightemitting substance containing layer 516.

A structure provided with a light emitting element 518 in the space 507surrounded by the element substrate 510, the sealing substrate 504, andthe sealing member 505 is formed by attaching the sealing substrate 504to the element substrate 510 with the sealing member 505. The space 507includes a structure filled with the sealing member 505 besides astructure filled with an inert gas (nitrogen, argon, or the like).

An epoxy-based resin is preferably used for the sealing member 505. Amaterial used for these is desirably a material which does not transmitmoisture or oxygen as possible. As a material used for the sealingsubstrate 504, a plastic substrate made of FRP (Fiberglass-ReinforcedPlastics), PVF (polyvinyl fluoride), myler, polyester, acrylic, or thelike can be used besides a glass substrate or a quartz substrate.

As described above, a light emitting device having a light emittingelement formed by the present invention can be obtained.

A light emitting device shown in this embodiment mode can be implementedby freely combining the structures of the light emitting element shownin Embodiment Modes 1 to 3.

Embodiment Mode 5

An electronic device provided with a light emitting device according tothe present invention includes: a television device (also simplyreferred to as a television or a television receiver), a camera such asa digital camera or a digital video camera, a cellular phone unit (alsosimply referred to as a cellular telephone or a cellular phone), aportable information terminal such as a PDA, a portable game machine, amonitor for a computer, a computer, a sound reproducing device such as acar audio set, an image reproducing device provided with a recordingmedium such as a home game machine, and the like. Specific examplesthereof will be explained with reference to FIGS. 6A to 6E.

A television device shown in FIG. 6A includes a main body 8001, adisplay portion 8002, and the like. The light emitting device of thepresent invention can be applied to the display portion 8002. As aresult, it enables not only to flow a sufficient current at a lowvoltage, but also to obtain sufficient luminescence properties. Further,optical characteristics can be enhanced by freely designing a thickness.According to these, a television device driven at a lower drive voltagecan be provided.

A portable information terminal device shown in FIG. 6B includes a mainbody 8101, a display portion 8102, and the like. The light emittingdevice of the present invention can be applied to the display portion8102. As a result, it enables not only to flow a sufficient current at alow voltage, but also to obtain sufficient luminescence properties.Further, optical characteristics can be enhanced by freely designing athickness. According to these, a portable information terminal devicedriven at a lower drive voltage can be provided.

A digital video camera shown in FIG. 6C includes a display portion 8201,a display portion 8202, and the like. The light emitting device of thepresent invention can be applied to the display portion 8202. As aresult, it enables not only to flow a sufficient current at a lowvoltage, but also to obtain sufficient luminescence properties. Further,optical characteristics can be enhanced by freely designing a thickness.According to these, a digital video camera driven at a lower drivevoltage can be provided.

A cellular telephone shown in FIG. 6D includes a main body 8301, adisplay portion 8302, and the like. The light emitting device of thepresent invention can be applied to the display portion 8302. As aresult, it enables not only to flow a sufficient current at a lowvoltage, but also to obtain sufficient luminescence properties. Further,optical characteristics can be enhanced by freely designing a thickness.According to these, a cellular telephone driven at a lower drive voltagecan be provided.

A portable television device shown in FIG. 6E includes a main body 8401,a display portion 8402, and the like. The light emitting device of thepresent invention can be applied to the display portion 8402. As aresult, it enables not only to flow a sufficient current at a lowvoltage, but also to obtain sufficient luminescence properties. Further,optical characteristics can be enhanced by freely designing a thickness.According to these, a portable television device driven at a lower drivevoltage can be provided. Further, the light emitting device of thepresent invention can be applied to various types of portable televisiondevices such as a small-sized television device incorporated in aportable terminal such as a cellular telephone, a medium-sizedtelevision device which is portable, and a large-sized television device(for example, 40 inches or more).

As described above, an electronic device driven at a lower drive voltagecan be provided according to the light emitting device of the presentinvention which enables not only to flow a sufficient current at a lowvoltage, but also to obtain sufficient luminescence properties and toenhance optical characteristics by freely designing a thickness.

Embodiment 1

In this embodiment, a manufacturing method and a measurement result ofelement characteristics of a light emitting element (a structure inwhich a buffer layer is provided to be in contact with a first electrode(anode)) having an element structure shown in Embodiment Mode 1, inwhich a buffer layer, a hole transporting layer, a light emitting layer,an electron transporting layer, an electron injecting layer, and asecond electrode are sequentially stacked from the first electrode sidewill be shown in FIGS. 7 to 11.

First, a first electrode 701 of the light emitting element is formed ona substrate 700 as shown in FIG. 7. In this embodiment, the firstelectrode 701 serves as an anode. The first electrode 701 is made of ITOwhich is a transparent conductive film and formed to have a thickness of110 nm by a sputtering method. As a sputtering method used herein, abipolar sputtering method, an ion beam sputtering method, or an oppositetarget sputtering method, and the like are given. The size of theelectrode was set to be 2 mm×2 mm.

Subsequently, a light emitting substance containing layer 702 is formedon the first electrode (anode) 701. In this embodiment, the lightemitting substance containing layer 702 has a stacked structure of abuffer layer 704, a hole transporting layer 706, a light emitting layer705, an electron transporting layer 707, and an electron injecting layer708 from the first electrode side, and especially, has a feature thatthe light emitting substance containing layer 702 is made of a compositematerial for a light emitting element fowled by aromatic hydrocarboncontaining at least one vinyl skeleton and metal oxide is used for thebuffer layer 704.

A substrate provided with the first electrode 701 is fixed to asubstrate holder of a vacuum deposition system which is commerciallyavailable so that a surface, on which the first electrode 701 is formed,is placed downward. A compound (DPVBi) represented by a structuralformula (I) in Embodiment Mode 1 as aromatic hydrocarbon containing atleast one vinyl skeleton is put in one of evaporation sources providedinside the vacuum deposition system, and molybdenum oxide as metal oxideis put in the other evaporation source to form the buffer layer 704 tohave a thickness of 120 nm by a co-evaporation method using resistanceheating. A weight ratio of DPVBi to molybdenum oxide in the buffer layer704 which is formed here was set to be 1:0.5 (molar ratio is set to be1:1.8) (=DPVBi:molybdenum oxide).

Next, the hole transporting layer 706 is formed by a material superiorin a hole transporting property. Here, as the material superior in ahole transporting property, NPB is formed to have a thickness of 10 nmby a deposition method using resistance heating.

Then, the light emitting layer 705 is formed. In the light emittinglayer 705, holes and electrons are recombined to emit light. Here, Alq₃and coumarin 6 are formed to have a thickness of 37.5 nm by aco-evaporation method similar to the buffer layer. A weight ratio ofAlq₃ to coumarin 6 was set to be 1:0.01 (molar ratio is set to be1:0.013) (=Alq₃: coumarin 6). According to this, coumarin 6 is includedin a layer made of Alq₃ by being dispersed.

Then, the electron transporting layer 707 is formed. As the electrontransporting layer 707, Alq₃ is formed to have a thickness of 37.5 nm bya deposition method similar to the hole transporting layer 706.

Then, the electron injecting layer 708 is formed. As the electroninjecting layer 708, LiF is formed to have a thickness of 1 nm by adeposition method similar to the hole transporting layer 706.

As described above, after forming the light emitting substancecontaining layer 702 having a stacked structure, a second electrode 703serving as a cathode is formed by a sputtering method or a depositionmethod. In this embodiment, aluminum is formed on the light emittingsubstance containing layer 702 to have a thickness of 200 nm by asputtering method to obtain the second electrode 703.

Accordingly, a light emitting element of the present invention isformed. In this embodiment, although a case where the first electrodeformed on the substrate is made of an anode material and serves as ananode is explained, the present invention is not limited to this, andthe first electrode can be made of a cathode material and made to serveas a cathode. However, in this case (a case of reversing the anode andcathode), a stacking sequence of the light emitting substance containinglayer is reversed from a case shown in this embodiment. Further, in thisembodiment, the first electrode 701 is a transparent electrode and has astructure in which light generated in the light emitting layer 705 ismade to be emitted from the first electrode 701 side; however, thepresent invention is not limited to this, and a structure in which lightis made to be emitted from the second electrode 703 side can also beemployed by selecting a suitable material for ensuring transmittance.

Next, a measurement result of element characteristics of a lightemitting element shown in FIG. 7 is indicated by plots represented bywhite circles (light emitting element (2)) in FIGS. 8 to 11.

In current-voltage characteristics shown in FIG. 8, a current ofapproximately 1.0 mA flowed when a voltage of 6V was applied. From thisresult, a state is continued where a current is injected sufficiently inthe light emitting substance containing layer 702 of the light emittingelement even in a case of a low voltage by providing the buffer layer704 of the present invention.

In luminance-voltage characteristics shown in FIG. 9, luminance ofapproximately 1000 cd/m² was obtained when a voltage of 6V was applied.From this result, it is revealed that a favorable effect can be obtainedwith respect to luminance characteristics to a voltage by providing thebuffer layer 704 of the present invention.

In current efficiency-luminance characteristics shown in FIG. 10,current efficiency was approximately 11.5 cd/A in a case where luminanceof 100 cd/m² was obtained. In luminance-current density characteristicsshown in FIG. 11, luminance of approximately 10000 cd/m² was obtained ina case where current density was 100 mA/cm². From these results, it canbe said that holes and electrons exist in a favorable balance, which isa state where recombination can be conducted effectively, in the lightemitting substance containing layer 702 of the light emitting element.

Comparative Example 1

In contrast, measurement was conducted with respect to elementcharacteristics of a light emitting element (referred to as a lightemitting element (1)) manufactured by using DPVBi as aromatichydrocarbon containing at least one vinyl skeleton for the buffer layer704 in the element structure which is measured in Embodiment 1. Thelight emitting element (1) which is measured here is manufactured so asto have the same structure as the light emitting element shown in FIG. 7of Embodiment 1 except for the buffer layer 704. Specifically, a firstelectrode: ITO (110 nm), a buffer layer: DPVBi (120 nm), a holetransporting layer: NPS (10 nm), a light emitting layer: Alq₃+coumarin 6(37.5 nm), an electron transporting layer: Alq₃ (37.5 nm), an electroninjecting layer: LiF (1 nm), and a second electrode: Al (200 nm) arestacked in this order. The result is represented by plots indicated byblack circles (light emitting element (1)) in FIGS. 8 to 11.

In current-voltage characteristics shown in FIG. 8, a voltage ofapproximately 24 V was necessary to be applied in order to flow acurrent of approximately 1.0 mA. It can be said that a current does noteasily injected into a light emitting substance containing layer of thelight emitting element compared with the light emitting element (2) inwhich the same amount of current can flow by applying a voltage ofapproximately 6 V.

In luminance-voltage characteristics shown in FIG. 9, a voltage ofapproximately 25 V is necessary to be applied to obtain luminance ofapproximately 1000 cd/m². It is revealed that luminance characteristicsof an element is influenced on a state where a current does not easilyinjected into a light emitting substance containing layer 702 of thelight emitting element compared with the light emitting element (2) inwhich the same luminance can be obtained by applying a voltage ofapproximately 6 V.

In current efficiency-luminance characteristics shown in FIG. 10,current efficiency was approximately 3.0 cd/A in a case where luminanceof 100 cd/m² was obtained. In luminance-current density characteristicsshown in FIG. 11, luminance of only approximately 5000 cd/m² wasobtained in a case where current density was 100 mA/cm². It can be saidthat these results are caused by a state where holes which are necessaryfor recombination are not injected sufficiently because holes andelectrons do not exist in a favorable balance in a layer including thelight emitting element in the light emitting element (1).

From these results, according to the light emitting element of thepresent invention, a hole injecting property in a light emittingsubstance containing layer can be enhanced by providing a buffer layermade of a composite material for a light emitting element formed byaromatic hydrocarbon containing at least one vinyl skeleton and metaloxide; therefore, a drive voltage of the light emitting element can bereduced and element characteristics such as luminance characteristicscan be enhanced.

This application is based on Japanese Patent Application serial No.2005-147967 field in Japan Patent Office on May 20, 2005, the entirecontents of which are hereby incorporated by reference.

1. A light emitting device comprising: an anode; a cathode; a bufferlayer provided between the anode and the cathode; and an electrongenerating layer provided between the anode and the cathode, wherein thebuffer layer comprises an aromatic hydrocarbon including at least onevinyl skeleton and comprises a metal oxide, wherein the buffer layer isin contact with the cathode, and wherein the metal oxide is one selectedfrom the group consisting of molybdenum oxide, vanadium oxide, rutheniumoxide, and rhenium oxide.
 2. The light emitting device according toclaim 1, wherein the electron generating layer is disposed over theanode, and wherein the buffer layer is disposed over the electrongenerating layer.
 3. The light emitting device according to claim 1,wherein the aromatic hydrocarbon including at least one vinyl skeletonis represented by any one of the following structural formulas (1)-(5),


4. The light emitting device according to claim 1, wherein the aromatichydrocarbon including at least one vinyl skeleton has hole mobility of1×10⁻⁶ cm²/Vs or more.
 5. The light emitting device according to claim1, wherein the electron generating layer comprises a substance selectedfrom the group consisting of an electron transporting substance and abipolar substance, and comprises a substance selected from the groupconsisting of alkali metal, alkaline earth metal, alkali metal oxide,alkaline earth metal oxide, alkali metal fluoride and alkaline earthmetal fluoride.
 6. The light emitting device according to claim 1,wherein the light emitting device is incorporated into an electronicdevice selected from the group consisting of a camera such as a digitalcamera and a digital video camera, a cellular phone, a portableinformation terminal, a portable game machine, a monitor for a computer,a computer, a laptop personal computer, a car audio set, an imagereproducing device, a car navigation system, a television device andportable television device.
 7. The light emitting device according toclaim 1, wherein the buffer layer serves as a barrier film.
 8. The lightemitting device according to claim 1, further comprising a second bufferlayer, wherein the second buffer layer is in contact with the electrongenerating layer.
 9. A light emitting device comprising: an anode; acathode; a light emitting layer interposed between the anode and thecathode; a buffer layer provided between the anode and the cathode; andan electron generating layer provided between the anode and the cathode,wherein the buffer layer comprises an aromatic hydrocarbon including atleast one vinyl skeleton and comprises a metal oxide, wherein the bufferlayer is in contact with the cathode, and wherein the metal oxide is oneselected from the group consisting of molybdenum oxide, vanadium oxide,ruthenium oxide, and rhenium oxide.
 10. The light emitting deviceaccording to claim 9, wherein the electron generating layer is disposedover the anode, and wherein the buffer layer is disposed over theelectron generating layer.
 11. The light emitting device according toclaim 9, wherein the aromatic hydrocarbon including at least one vinylskeleton is represented by any one of the following structural formulas(1)-(5),


12. The light emitting device according to claim 9, wherein the aromatichydrocarbon including at least one vinyl skeleton has hole mobility of1×10⁻⁶ cm²/Vs or more.
 13. The light emitting device according to claim9, wherein the electron generating layer comprises a substance selectedfrom the group consisting of an electron transporting substance and abipolar substance, and comprises a substance selected from the groupconsisting of alkali metal, alkaline earth metal, alkali metal oxide,alkaline earth metal oxide, alkali metal fluoride and alkaline earthmetal fluoride.
 14. The light emitting device according to claim 9,wherein the light emitting device is incorporated into an electronicdevice selected from the group consisting of a camera such as a digitalcamera and a digital video camera, a cellular phone, a portableinformation terminal, a portable game machine, a monitor for a computer,a computer, a laptop personal computer, a car audio set, an imagereproducing device, a car navigation system, a television device andportable television device.
 15. The light emitting device according toclaim 9, wherein the buffer layer serves as a barrier film.
 16. Thelight emitting device according to claim 9, further comprising a secondbuffer layer, wherein the second buffer layer is in contact with theelectron generating layer.
 17. A light emitting device comprising: ananode; a cathode; a layer containing a light emitting substance, thelayer being interposed between the anode and the cathode; a buffer layerprovided between the anode and the cathode; and an electron generatinglayer provided between the anode and the cathode, wherein the layerincludes at least one of a light emitting layer, a hole transportinglayer, an electron transporting layer, and an electron injecting layer,wherein the buffer layer comprises an aromatic hydrocarbon including atleast one vinyl skeleton and comprises a metal oxide, wherein the bufferlayer is in contact with the cathode, and wherein the metal oxide is oneselected from the group consisting of molybdenum oxide, vanadium oxide,ruthenium oxide, and rhenium oxide.
 18. The light emitting deviceaccording to claim 17, wherein the electron generating layer is disposedover the anode, and wherein the buffer layer is disposed over theelectron generating layer.
 19. The light emitting device according toclaim 17, wherein the aromatic hydrocarbon including at least one vinylskeleton is represented by any one of the following structural formulas(1)-(5),


20. The light emitting device according to claim 17, wherein thearomatic hydrocarbon including at least one vinyl skeleton has holemobility of 1×10⁻⁶ cm²/Vs or more.
 21. The light emitting deviceaccording to claim 17, wherein the electron generating layer comprises asubstance selected from the group consisting of an electron transportingsubstance and a bipolar substance, and comprises a substance selectedfrom the group consisting of alkali metal, alkaline earth metal, alkalimetal oxide, alkaline earth metal oxide, alkali metal fluoride andalkaline earth metal fluoride.
 22. The light emitting device accordingto claim 17, wherein the light emitting device is incorporated into anelectronic device selected from the group consisting of a camera such asa digital camera and a digital video camera, a cellular phone, aportable information terminal, a portable game machine, a monitor for acomputer, a computer, a laptop personal computer, a car audio set, animage reproducing device, a car navigation system, a television deviceand portable television device.
 23. The light emitting device accordingto claim 17, wherein the buffer layer serves as a barrier film.
 24. Thelight emitting device according to claim 17, further comprising a secondbuffer layer, wherein the second buffer layer is in contact with theelectron generating layer.